Publication DE 27 19 823 C2 makes known a power split transmission, in which a hydrostatic transmission composed of a hydraulic pump and a hydraulic motor is disposed between a drive unit having a drive shaft and an output shaft. Furthermore, a hydrodynamic transmission which is composed of a hydrodynamic converter is connected in parallel via a spur gear stage. A further spur gear stage for connection to the output shaft is provided on the output side of the hydrodynamic transmission, wherein, with respect to the hydrodynamic transmission, the spur gear is connected to the output shaft of the hydrodynamic converter via an overrunning clutch.
Furthermore, publication DE 20 2005 021 249 U1 makes known a power split transmission for motor vehicles, in which a mechanical and a hydrostatic power split are disposed in parallel. In that case, a double clutch transmission is provided between a drive shaft and the mechanical power branch, and can be used to connect two transmission shafts. Constant-velocity devices are provided on each of the aforementioned transmission shafts to connect idler gears disposed on the transmission shafts to the particular transmission shaft in a form-locking manner. By connecting the idler gears in this manner, the power of the drive shaft is transmitted to a hollow shaft situated parallel thereto. A hydrostatic power branch is disposed inside aforementioned hollow shaft, via which a spur gear stage is connected to the drive shaft. The power from the mechanical branch and the hydrostatic branch are combined in a planetary gear assembly, the hydrostatic power branch being connected to the sun gear, and the hollow shaft of the mechanical power branch being connected to the ring gear. The planetary gears of the planetary gear assembly are connected to an output shaft to draw the output power from the power split transmission.
The disadvantage of the known prior art is that power losses result when a hydrodynamic power branch is used, due to the warming of the oil. When a mechanical power branch comprising spur gear stages is used, a certain amount of axial installation space is required, which is not available in many vehicles.